It is known to use power steering in road vehicles, e.g., electrical power assisted steering, commonly abbreviated as EPAS, in a road vehicle such as a car, lorry, bus or truck, wherein an electric motor assists a driver of the road vehicle by adding an assistive torque to e.g., a steering column or steering rack of the road vehicle.
It is further known to use advanced driver assistance systems, commonly abbreviated as ADAS, such as Lane Keeping Aid systems, commonly abbreviated as LKA systems, in order to help a road vehicle driver maintain the road vehicle in a desired lane. For LKA or lane centering systems where an EPAS is used, a steering wheel torque overlay, i.e., additional steering wheel torque on top of what would have been obtained by the base assist of the EPAS, is used for lateral position control.
However, the need for more advanced autonomous steering functions has put new requirements on current steering safety concepts. One example of such is commonly called Pilot Assist, commonly abbreviated as PA, which helps a driver to drive the vehicle within the road lane whilst at the same time maintaining a preselected time interval to a preceding vehicle.
Current safety mechanisms for driver assistance functions, such as PA functions, commonly rely on a driver to always hold his or her hands on the steering wheel. By holding the steering wheel, the driver will influence a wheel angle controller with a torque disturbance. When the driver is resting his or her hands on the steering wheel the torque disturbance depends on the biometrics of the driver as well as the numerous different ways different drivers might hold the steering wheel.
A Steer Torque Manager, commonly abbreviated as STM, is a component that includes a Driver In the Loop functionality, commonly abbreviated as DIL functionality, and a wheel angle controller. The DIL functionality usually relies on a torsion bar torque sensor to decide when and how to hand over control to the driver from the Pilot Assist- or Lane Keeping Aid-functionality and when the driver is to be treated as a disturbance that needs to be suppressed by the wheel angle controller. The DIL-functionality hands over control to the driver by limiting the torque output of the wheel angle controller, e.g., by scaling the output or by decreasing the maximum and minimum allowed overlay torque. The STM is commonly located in an EPAS supplier node, herein referred to as Power Steering Control Module, commonly abbreviated as PSCM.
Since a driver is required to keep his or her hands on the steering wheel the overall comfort and customer value of the PA functionality is largely decided by the DIL-functionality, which needs to handle two partly contradictory requirements. Firstly, the DIL-functionality and the wheel angle controller should be able to track a wheel angle request from the PA path while suppressing the torque disturbance from a driver that rests his or her hands on the steering wheel. Secondly, when the driver desires to take control of the vehicle, the DIL-functionality should comfortably hand over control to the driver, thereby assuring that the driver never experiences high torque in the steering wheel.
These two requirements are contradictory because the first requirement relies on being able to suppress steering wheel torque disturbances, while the second requirement means that the driver should never experience a large counteracting torque from the wheel angle controller.
When a driver rests his or her hands on the steering wheel, whilst shoulders and arms are rested, and whilst an advanced driver assistance system such as PA is active, steering wheel torques in the magnitude of 0.5-1.5 Nm could unintentionally be caused by a driver. Such unintentional disturbances from a driver will normally cause small movements of the steering wheel which in turn will cause short torque pulsations when counteracted by the wheel angle controller in the STM as it tries to compensate for the disturbance. In some cases, the driver will also feel the resulting lateral motion as this also may cause torque disturbances at a motor controller of a steering system of the road vehicle, e.g., up to approximately 0.15 Nm.
This is due to the fact that the EPAS assistance is high, which means that even low levels of torque on the steering wheel will cause movements. A torque request from the wheel angle controller is derived using an angle error, which means that the angle error happens first before the wheel angle controller increases the torque output and compensates for the EPAS assistance disturbance.
Thus, there is a need for improved solutions which are able to simultaneously handle both of the above requirements while facilitating fulfilment of high Automotive Safety Integrity requirements.